The longterm goal of the last 20 years of study of the somatic sensory system in the prosimian primate, Galago, is an understanding of the role of the primary (SI) and secondary (SII) cortical projections areas in tactile discrimination capacity. In the adult Galago, neurons in the ventroposterior (VP) thalamus project to either SI or SII, whereas in the infant, many VP neurons extend collaterals to both SI and SII. In adult anthropoids, VPL projects only to SI, and SII receives tactile input indirectly through SI. Input from the hairy (H) and glabrous (G) hand projects to segregated and separately organized regions of SI and SII in the adult Galago. This contrast with SI and SII areas in Old and New World anthropoids, in which the major G input obscures the minor H input from the hand. Hence it is proposed: 1) that neither place (SI or SII) nor modality (H or G) are important in directing initial outgrowth early in development of all primates; 2) that VP neurons extend collaterals to the eventual G and H zones of SI and SII; and that peripheral and cortical activity influences the final projection pattern. In Galago, I collateral elimination may occur on the basis of submodality, whereas in anthropoids, on the basis of cortical locus (all SII collaterals are eliminated). To examine predictions based on such a proposal, studies of C and H input to SI and SII in infant and adult Galago are proposed: 1) electrophysiological studies will examine the size, submodality and topographic organization of input from the hand to areas 3b and 1 of SI and of dorsal and ventral zones of SII in infants and adults; 2) anatomical studies using multiple fluorescent dyes will examine the pattern of projections to these zones of SI and SII (which are normally segregated into G and H areas) in normal infants and adults; 3) studies using long-term fluorescent labels will determine the temporal 1 pattern of maintained VP projections in normal and experimental animals (with removal of H input, SI or SII as infants or adults); 4) and discrimination tasks will be presented to evaluate tactile capacity in animals, with expansions and/or duplications of G input in SI or SII, produced by such manipulations. These studies have implications for mechanism: 1) mediating normal cortical and behavioral development; 2) the recovery of function after early brain damage; 3) the morphological basis for the "unmasking" of input after peripheral lesions in adults; and 5) the developmental mechanisms leading to the evolution of increased cortical size and complexity in primates.